FIGS. 8, 9 and 10 are views for explaining steps of installing a battery in a conventional battery installation device. A mobile phone set 1 is so designed that a battery 3 is housed in a battery housing part 2 provided in an area where a back lid is opened, and then, the battery housing part is covered with the back lid 4. In the illustrated example, because the battery 3 is heavy, a projection (not shown) provided at a distal end portion of the battery 3 is inserted (in a direction of an arrow mark A in FIG. 8) into a recess (not shown) provided in a body of the mobile phone set 1 to fix the distal end portion, and then, a rearward portion of the battery 3 is received (in a direction of an arrow mark B in FIG. 9) in the battery housing part 2. Thereafter, by sliding a lock member 5 of a sliding type which is provided in the body of the mobile phone set 1 in a direction of an arrow mark C in FIG. 9, a projected portion 3a provided in the rear end portion of the battery 3 (See FIG. 8) is pressed, thereby to lock the battery 3 to the body of the mobile phone set 1.
Thereafter, the back lid 4 is fitted (in a direction of an arrow mark D in FIG. 9) to a back side of the body of the mobile phone set 1, and by sliding the back lid 4 with force in a direction of an arrow mark E in FIG. 10, retaining pieces 4a, 4b which are provided in a projecting manner at a distal end of the back lid 4 formed of resin are flexed and engaged with engaging recesses 6, 7 in the body of the mobile phone set 1, and the back lid 4 is fixed so as not to be easily detached from the body of the mobile phone set 1.
In case where the conventional mobile phone set as described above is dropped, the battery 3 which is locked will not be detached from the body of the mobile phone set 1. However, the back lid 4 may be detached from the body of the mobile phone set 1 with a shock of the drop, because the back lid 4 is simply fitted to the back side of the body and further, the retaining pieces 4a, 4b have been engaged with the engaging recesses 6, 7. There is such a problem that after the back lid 4 is repeatedly fitted and detached, the above described retaining pieces 4a, 4b may be deteriorated, and the back lid 4 will be unable to be fixed to the body of the mobile phone set 1.
Although this problem can be solved by providing a lock mechanism for locking the back lid 4 to the body of the mobile phone set 1, it would be difficult, due to limitations of space, to provide both the lock mechanism for locking the battery 3 and the lock mechanism for locking the back lid 4 in a small casing of the mobile phone set, and this will lead to a rise of production cost. There is a further problem that a work for exchanging the battery is annoying, because the two lock mechanisms must be operated.
In some of the conventional mobile phone set, the back lid is provided with the lock mechanism, and the battery is simply incorporated in the body of the mobile phone set 1. However, such a state that the heavy battery is not locked inside the body of the mobile phone set may have a detrimental effect on reliability of the mobile phone set, and may be unfavorable.